1. Field of the Invention
The present invention relates to an electronic apparatus, and more particularly, to a half-bridge converter having an improved power factor.
2. Description of the Related Art
In general, a rectifier receives an AC power and outputs a DC power. A DC-DC converter converts a DC current to an AC current, boosts or steps down the AC current, and then rectifies the AC current. Among DC-DC converters, there are a pulse width modulation converter, a quasi-resonant converter, and a multi-resonant converter.
Since electronic apparatuses change sensitively to the ripple characteristic of a DC power voltage, studies for reducing the ripples in an output DC power have been actively carried out. For this purpose, a condenser-input rectifying method is used to convert an AC power to a DC power. In this method, a full-bridge rectifier, consisted of diodes and a condenser having a large capacitance, is included in an input terminal.
However, if a condenser having a large capacitance is used, the power factor of an AC input terminal is lowered to 0.5-0.6 due to the charging current of the condenser. This is not preferable because if the power factor is lowered, the efficiency of the input terminal is degraded.
FIG. 1a is a diagram showing an embodiment of a prior art converter, and FIG. 1b is a waveform diagram for explaining the operation of the prior art converter.
Referring to FIG. 1a, during half a cycle, diodes D1 and D4 conduct, and during the remaining cycle, diodes D2 and D3 conduct, so the waveform of the output is as shown in FIG. 1b. 
In interval [0, t1] where the output voltage rises, a charging current flows because a supplied voltage is greater than a voltage (Vc) being charged to the condenser (Cs). On the contrary, in interval [t1, t2] where the output voltage falls, the condenser discharges, and its time constant corresponds to the product of the load (RL) and the capacitance (Cs) of the condenser. The charging interval [0, t1] is much shorter than the interval [t1, t2]. Here, [a, b] represents an interval from a to b. However, due to the charging current of the condenser (Cs) which is used to reduce the ripples in the output voltage, the power factor of the converter shown in FIG. 1a is lowered. Also, since the input current flows only when the input voltage (Vi) is greater than the condenser voltage (Vc), the input current flows in pulse shape around the maximum value of the input voltage and therefore includes more harmonic components than a sinusoidal current.
To solve the problem, a power factor improving unit is introduced in the input part.
In the boost converter of discontinuous-current mode which is commonly used, an input current naturally follows the sinusoidal shape of the input voltage. However, only when the output voltage of the power factor improving unit is much greater than the maximum value of an the input voltage, the power factor is improved. Meanwhile, if the output voltage of the power factor improving unit increases, voltages applied to semiconductor devices also increase. Therefore, in order to stand this voltage stress, a semiconductor switch having a higher rated voltage should be used. On the other hand, since in the semiconductor switch having a higher rated voltage the conducting resistance is also high, the conducting loss also increases. If the conducting loss increases, the efficiency of the entire system is degraded.
Therefore, it is desirable to provide a converter which reduces the conducting loss in a semiconductor device, improves the power factor of the input terminal, and thus improves the efficiency of the entire system.
To solve the above problems, it is an objective of the present invention to provide a converter having a high power factor in which the voltage of the switching unit of the converter is fed back to the input terminal of the converter so that the conducting loss occurring in the switching unit decreases and the power factor of the input terminal is improved.
To accomplish the objective of the present invention, there is provided a half-bridge converter having a bridge diode unit transmitting energy to a voltage smoothing capacitor; the voltage smoothing capacitor for storing energy provided by the bridge diode unit; and a switching unit having two switches serially connected between both ends of the voltage smoothing capacitor, in which the power factor improving unit for feeding the voltage of the common connection point of the switches forming the switching unit back to the common connection point of the input capacitors so as to change the input current according to the magnitude of the input voltage is further included.
It is preferable that the power factor improving unit has a feedback inductor for feeding the voltage of the common connection point of the switches forming the switching unit back to the common connection point of the input capacitors; and a coupling inductor of which a first winding is connected between one end of the input voltage and one end of the bridge diode unit, a second winding is connected between the other end of the input voltage and the other end of the bridge diode unit, and the first winding and the second winding are coupled.
It is preferable that the power factor improving unit has a coupling inductor of which a first winding is connected between one end of the input voltage and one end of the bridge diode unit, a second winding is connected between the other end of the input voltage and the other end of the bridge diode unit, and the first winding and the second winding are coupled; a first inductor for linearly changing the current between one end of the input voltage and the first winding of the coupling inductor; and a second inductor for linearly changing the current between the other end of the input voltage and the second winding of the coupling inductor, in which the common connection point of the switches forming the switching unit is connected to the common connection point of the input capacitors.
It is preferable that the power factor improving unit has a feedback inductor for feeding the voltage of the common connection point of the switches forming the switching unit back to the common connection point of the input capacitors.
It is preferable that the power factor improving unit has a first inductor for linearly changing the current between one end of the input voltage and one end of the bridge diode unit; and a second inductor for linearly changing the current between the other end of the input voltage and the other end of the bridge diode, in which the common connection point of the switches forming the switching unit is connected to the common connection point of the input capacitors.
Accordingly, the power factor of the input terminal is improved in the half-bridge converter according to the present invention.